Abstract In recent years the waste water ministerial [604439]
Abstract —In recent years the waste water ministerial
regulations have led to a constant ascend in the purification
performance demanded of waste water treatment plants.
Because of this, the number of waste water treatment plants
has been maturing, and technical complexity has also been
growing. In order to hold the connected rising costs of capital
expenditure and operation within bounds, sagacious process
technology solutions have to be found. Besides having a deeper
understanding of the individual pro cesses, it is indispensable to
consider the entire waste water treatment plant as a whole.
Most treatment plants consist of a mechanical and biological
waste water purification, sludge treatment and gas utilization.
In three of these four stages, namely in the preliminary and
secondary clarification of the waste water and in the
thickening and dewatering of the sludge, the processes for
solids/liquids separation are of crucial importance.
The efficiency of the solids/liquids separation is mainly
influenced by the properties of the sludge.
Index Terms
microorganisms, tr eatment process
I. I
NTRODUCTION
The Activated Sludge (AS) process was expanded as an
intermittent to biological filters, and is particularly
beneficial for large populations where land is at a premium.
More recent research however, has shown that the process
can be acted in many different modes, manufacturing it a
more flexible process than biological filtration [1]. The
Activated Sludge process is apeltednatural biological
treatment process. It is a complex mix of microbiology and
biochemistry importing many different sorts of bugs. In the
Activated Sludge Plant (ASP) bacteria secrete adhesive
substances that clothe the minute particles carried in sewage.
The particles stick together to form flocks of gel-like
material, creating a support on, and in which, the bugs exist.
This is the chocolate-brown colored activated sludge. The
activated sludge is aerated to dissolve oxygen which allows
the organic matter (BOD) to be utilized by the bugs. The
organic matter, or food, cohesions to the activated sludge.
The oxygen dissolved in the water allows the bugs to usage
the food (BOD) and also to ch ange the ammonia to nitrate.
The tank should be big sufficient to allow sufficient contact
time (retention time) between the sewage and the activated
sludge for all the chemical changes to take place [2], [3].
Manuscript received June 1, 2013; revised September 3, 2013.
B. Ahansazan, H. Afrashteh, N. Ah ansazan, and Z. Ahansazanare with
Ministry of Science, Research and Technology of Iran. Valiasr Technical
College of Tehran, Iran (e-mail: [anonimizat]).
II. B
IOLOGICAL
P
ROCESSES
A. Biological Treatment by Activated Sludge
Wastewater achieves from two major sources: as human
sewage and as process waste from making industries. In the
UK, the total volume of wastewater from industry is about 7
times that of household sewage. If untreated, and dismissed
directly to the environment, the receiving waters would
become polluted and water-borne illness would be widely
diffused. In the early years of the twentieth century the
method of biological treatment was invented, and now
forms the basis of wastewater treatment worldwide[1]. It
simply imports confining naturally occurring bacteria at
very much higher concentrations in tanks. These bacteria,
together with some protozoa and other microbes, are
collectively concerned to as activated sludge. The construct
of treatment is very simple . The bacteria remove small
organic carbon molecules by ‘ea ting’ them. As a result, the
bacteria flourish, and the wastewater is cleansed. The
treated wastewater or effluent can then be discharged to
arriving waters – normally a river or the sea [2].
Whilst the concept is very simple, the control of the
treatment process is very abstruse, because of the large
number of variables that can affect it. These include changes
in the combination of the bacterial flora of the treatment
tanks, and changes in the sewage passing into the plant [3].
The influent can show variations in flow rate, in chemical
composition and pH, and temperature [4]. Many urban
plants also have to contend with surge flows of rainwater
following storms. Those plants receiving industrial
wastewater have to cope with rebellious chemicals that the
bacteria can decline only ve ry slowly, and with toxic
chemicals that debar the functi oning of the activated sludge
bacteria. High concentrations of toxic chemicals can output
a toxic shock that kills the bacteria. When this happens the
plant may transit untreated effluent direct to the
environment, until the dead bacteria have been eliminated
from the tanks and new bacterial ‘seed’ introduced[2] ,[5].
Globally, the combination of effluents dismissed to
receiving waters is regulated by the national environment
agencies. In Europe the regula tory regulation is the Urban
Waste Water Treatment Directive (1991) and the more
recent Water Framework Directiv e (2000). In the USA, the
Environmental Protection Agency (EPA) certifies
compliance with the Clean Water Act (1977). The law is
concerned with the forbidding of pollution, and therefore
sets concentration limits on dissolved organic carbon (as
BOD or COD), nitrogen and phosphates – which cause
eutrophication in achieving waters. It also attempts to bound
the separated of known toxic chemicals by setting allowable
concentration limits in the e ffluent [2],[4]. Recently, in
distinction that effluents contain unknown toxic chemicals,
a more practical approach to adjustment is being introduced
Activated Sludge Process Overview International Journal of Environmental Science and Development, Vol. 5, No. 1, February 2014
81
—Activated ludge, astewater,
s w
DOI: 10.7763/IJESD.2014.V5.455
B. Ahansazan, H. Afrashteh, N. Ahansazan, and Z. Ahansazan
in Europe, using Direct Toxicity Assessment (DTA) tests. In
the US these have been in usage for many years and are
known as Whole Effluent Toxicity (WET) tests. These tests
are used to measure the toxi c factors of effluents on
representative organisms from the receiving waters. Any
toxicity observed in the effluents will obviously have been
extant in the sewage entering the plant. Surprisingly, direct
toxicity assessment of influents to wastewater treatment
plants that could hit on the functioning of the bioprocesses
is not yet included in legislation [2],[3],[6].
B. The Nature and Composition of Wastewater
Domestic sewage is constituted largely of organic carbon,
either in solution or as partic ulate matter. About 60% is in
particulate form, and of this, slightly under a half is large
adequate to settle out of suspension. Particles of 1nm to
100μm remain in colloidal suspension and pending
treatment become adsorbed on to the flocks of the activated
sludge [2], [3].
The bulk of the organic matter is easily biodegradable,
including of proteins, amino acids, peptides, carbohydrate,
fat and fatty acids. The average carbon to nitrogen to
phosphorus ratio (or C: N: P ratio) is diversely stated as
approx 100: 17: 5 or 100: 19: 6. This is close to the ideal for
the growth of the activated sludge bacteria. However,
industrial wastewaters are very much more variable in
combination. Those manufactured by the brewing, and pulp
and paper industries, for exampl e, are deficient in nitrogen
and phosphate. These nutrients need to be added therefore to
attain the correct ratio for microbial growth, and to allow
treatment to proceed op timally[2],[3 ],[5],[7].
C. Degradable and Non-Degradable Carbon
For control of the biological processes in a treatment
plant, it is essential to have some knowledge of the organic
strength, or organic load, of the influent wastewater. Three
different measures of this are usable, and they each have
their merits and weaknesses. The Total Organic Carbon
(TOC) is being analytic straightforward to measure. It
includes oxidation by combustion at very high temperatures
and module of the resultant CO2. However, TOC values
comprise those stable organic carbon combination that
cannot be broken down biologically [1]-[3].
Organic carbon can also be calculated by chemical
oxidation. The sample is heated in strong sulphuric acid
containing potassium dichromate, and the carbon oxidised is
specified by the amount of dichromate used up in the
reaction [2], [7]. The result is represent in units of oxygen,
rather than carbon, and the pr ocedure is referred to as the
Chemical Oxygen Demand (COD). Again it is an
analytically simple method. However, its weakness is that a
number of indomitable organic carbon combination that are
not biologically oxidisable, are contained in the value
obtained. Conversely, some aromatic combination,
including benzene, toluene, and some pyridines, which can
be broken down by bacteria, are only partly oxidised in the
COD method. Overall howeve r, COD will overrate the
carbon that can be removed by the activated
sludge[2],[4],[5],[8].
The current method used to define the biodegradable
carbon, is the 5-day Biological Oxygen Demand (BOD5).
This is a measure of the oxygen uptake over a 5-day period by a small ‘seed’ of bacteria when limited, in the dark, in a
bottle containing the wastewater. During this time the
biodegradable organic carbon is derived, and there is a
corresponding 4 reduction in the dissolved oxygen, as some
of the carbon is used for the aspiration of the bacteria [2],[3].
Rather unhelpfully, the biodegradable carbon, as in the
COD test, is represent in oxygen units. This is because the
test was originally reported to measure the oxygen
evacuation in receiving waters effected by the residual
degradable carbon in the effluent. Its main value is in
adjusting the composition of effluents from the treatment
water. For process management, where knowledge of the
organic loading of the influent is needed, BOD5 is of
feinted value, because of the 5 days necessary to make the
measurement. There are now m oves afoot to change the use
of BOD5 as a measure of influent strength, with a short-
term test (BODST), which can be bearer out over a
timescale of 30 minutes to several hours [2],[4],[6],[7].
The values received for BOD5 are always lower than
those for COD, for 2 reasons:
• Activated sludge bacteria cannot degrade some of the
combination oxidized chemically in the COD test.
• Some of the carbon eliminated during the BOD test is not
oxidized, but ends up in new bacterial biomass. So the BOD
is only measuring the biodegradable carbon that is really
oxidized by the bacteria [1],[2].
The ratio of BOD5/COD will appertain the composition
of the wastewater. For household sewage, and also the
wastewaters from the slaughterhouse, dairy, distillery and
rubber industries, the ratio is about 0.5 – 0.6. However, for
sewage leaving the treatment plant, it is closer to 0.2. This is
because the readily biodegradab le organic carbon has been
deleted during treatment, departing behind the compounds
that are not readily broken down by the bacteria – ‘hard’
BOD. These will be readily measured by chemical oxidation,
but will not be readily degraded and eliminated by the
bacteria in the BOD bottle [1]-[4].
III. T
HE
C
OMPOSITION OF
A
CTIVATED
S
LUDGE
A. Activated Sludge Bacteria
The activated sludge of the aeration laver of a wastewater
treatment works is a complex ecosystem of competing
organisms. The dominant organisms are the bacteria, of
which there may be 300 species ubiquitous. Bacteria are
amongst the smallest and most abundant living organisms.
Each comprises a single cell va rying in size from about 0.5
– 2 μm. On the outside, the cell is bounded by membranes
that adjust the inflow of ions and molecules from the
surrounding water. This, in turn is surrounded by a hardcell
wall, created of a sugar polymer. The interior of the cell
contains the cytoplasm and the thousands of different
chemicals whose reactions are regulated by enzymes. The
bacterial cell does not have a nucleus. Most bacteria are
orbicular, but some may be rod shaped or have a spiral form.
Filamentous bacteria contain lo ng chains of small bacterial
cells, sometimes surrounded by a tubular sheath, and can
reach lengths of 100 μm.[2],[3],[5].
Small molecular weight compounds spread into the
bacteria (ingestion) through th e cell wall. At the same time,
some larger complex molecules that have been synthesized International Journal of Environmental Science and Development, Vol. 5, No. 1, February 2014
82
within the bacteria, pass outwards. This process is referred
The secretions include slimes and gels that may bond the
bacteria together and also enzymes. The enzymes break
down large organic molecules into smaller monomers that
are small sufficient to be ingested [2].
The bacteria use the ingested molecules for the synthesis
of new molecules, in the process of growth. When they have
attained normal size, the bacterium divides into two, and the
process is repeated. If nutrie nt molecules are not limiting,
this results in progressive growth in the numbers of bacteria
[2].
The bacteria in a wastewater treatment plant included
both heterotrophy and autographs. The heterotrophic or
carbonaceous bacteria are the do minant group of organisms.
They are characterized by nutrition mainly on organic
carbon molecules rather than inorganic ones. By mutuality,
the autographs take in inorganic chemicals, and use these in
the synthesis of organic compounds. The nitrifying bacteria
that remove ammonia from th e wastewater are the most
significant of this group. Ther e are relatively few species of
autographs, and since they have low growth rates, they tend
to be out- emulated by the faster-growing
heterotrophy[2],[3],[5].
T
HE
B
IOLOGICAL
B T
REATMENT
Activated sludge is a suspended growth secondary
treatment process that primarily removes dissolved organic
solids as well as settle-able and non-settle-able suspended
solids. The activated slud ge itself includes of a
concentration of microorganisms and sludge particles that
are naturally found in raw or settled wastewater. These
organisms are cultivated in aeration tanks, where they are
provided with soluble oxygen and food from the effluent.
The term “activated” comes from the fact that the particles
are teeming with bacteria, fungi, and protozoa.[1],[2],[9].
Like in most other wastewater treatment plants, when
wastewater enters an activated sludge treatment facility the
preliminary treatment processe s eliminate the coarse or
heavy inorganic solids (grit) and other debris, such as rags,
and boards. Primary clarifiers (if they are provided) remove
much of the floatable and settle able organic material. The
activated sludge process can treat either primary
clarified.[2],[5],[9].
Wastewater or raw wastew ater immediately from the
preliminary treatment processes. As the wastewater enters
the aeration basin, the activat ed sludge microbes use the
solids in the wastewater. After the aeration basin, the
wastewater solids and microo rganisms are separated from
the water through gravity settling which occurs in a
secondary clarifier. The settled solids and microorganisms
are pumped back to the front of the aeration basin, while the
clarified water flows on to the next component
[1],[2],[9],[10].
Scheme of the activated sludge system is shown in Fig. 1.
A. Providing Controllable Influent Feeding
The feeding of wastewater to activated sludge systems
must be controlled in a manner that certify even loading to all of the aeration basins in operation. Well-designed flow
splitter boxes should be accommodate into the front of the
aeration basin and they should be checked periodically to
ensure that the flow distribution is split as intended [2]. In
some situations, it is desirable to feed wastewater
throughout various points in the aeration basin. This is
known as step feeding. Step feeding is one method of
relieving the high oxygen demand that can befall where the
influent flow and RAS enter the aeration basin. However, a
downside to step feeding is th at some of the soluble solids
in the influent may pass through the aeration basin too
rapidly, and show up in the effluent as BOD [1],[9],[10].
B. Maintaining Proper Disso lved Oxygen and Mixing
Levels
Activated sludge microorgani sms need oxygen as they
oxidize wastes to receive energy for growth. Insufficient
oxygen will slow down or kill off aerobic organisms, make
facultative organisms work less efficiently and ultimately
lead to the production of the foul-smelling by-products of
anaerobic analysis. As the mass of organisms in an aeration
tank increment in number, the amount of oxygen needed to
support them also increases. Hi gh concentrations of BOD in
the influent or a higher infl uent flow will increment the
activity of the organisms and thus augmentation the demand
for oxygen. Sufficient oxygen must always be maintained in
the aeration tank to ensure complete waste stabilization.
This means that the level of oxygen in the aeration tank is
also one of the critical controls available to the operator. A
minimum dissolved oxygen (D.O.) level of 1.0 mg/L is
counsel in the aeration tank for most basic types of activated
sludge processes. Maintaining > 1.0 mg/L of D.O.
contributes to establishing a favorable environment for the
organisms, which produces the desired type of organism and
the eligible level of activity. If the D.O. in the aeration tank
is allowed to drop too low for long periods, undesirable
organisms, such as filamentous type bacteria may expand
and overtake the process. Conver sely, D.O. levels that are
allowed to rise too high can cause problems such as flock
particles being floated to the surface of the secondary
clarifiers. This problem is particularly common pending
cold weather. For these reas ons it is important that the
proper dissolved oxygen levels be maintained in the aeration
basin. This needs routine monitoring by the system operator
using a D.O. meter[8]-[10].
Fig. 1. Plant layout[1].
International Journal of Environmental Science and Development, Vol. 5, No. 1, February 2014
83
to as secretion [2],[3].
ASIS OF ASTEWATER
W IV.
C. Describe the Characteristics of Healthy Activated
Sludge
The color of healthy activated sludge is tan to brown. It
would have anterrestrial odor. During a 30 minute settling
test, the settled sludge volume would be 200-300 mL/L. The
SVI would be 80-150. The supernatant would be clear with
little or no flock particles. Sludge age for formal systems
would be 3-10 days and 15-30 days for extended aeration
systems [1]-[3].
1) Discuss the characteristics of young and old
activated sludge:
a) Young sludge
Young sludge includes of sludge which has not yet
reached a high enough sludge age to be most impressive in a
particular activated sludge process. Billowing whitish foam
is an index that the sludge age is too low. Young sludge will
often have poor settling characteristics in the clarifier, and
can leave straggler flock in the clarifier effluent. Young
sludge is often affiliate with a high F/M. To correct for
young sludge it is needful to reduction wasting rates. This
will increment the amount of so lids under aeration, detract
the F/M ratio, and increase the sludge age[2],[4].
b) Old Sludge
Old sludge comprises of sludge in which the sludge age is
too high to be most efficient in a particular activated sludge
process. Dark brown foam and a somewhat greasy or
scummy appearance is an indicator of old sludge. Settling in
the clarifier is fast, but pin flock can be existent in the
effluent and the effluent is hazy. Old sludge is often
associated with a low F/M ratio. To accurate for old sludge,
it is necessary to increment wasting rates and return less
sludge to the aeration basin. This will decrease the amount
of solids under aeration, augmentation the F/M ratio and
decrease the sludge age [2],[4].
D. Controlling the RAS Pumping Rate
The amount of time that solids expend on the bottom of the
secondary clarifier is a function of the RAS pumping rate.
The settled microorganisms and solids are in a embitter
condition as long as they remain in the secondary clarifier.
If sludge is allowed to remain in a secondary Separator too
long it will begin to float to the surface of the clarifier due
to nitrogen gas assert during the biological process ofde-
nitrification (rising sludge)[9], [10]. Monitoring and
controlling the depth of the sludge blanket in the secondary
Separator and the concentration of solids in the RAS are
significant for the proper operation and control of the
activated sludge system. A sludge settle-ability test, known
as a settle meter, can be used to indication the rate of sludge
settling and compaction.
This information is used to
detest
appropriate RAS pumping rates. Typically, RAS
pumping rates of between 25% and 150% of the influent
flow are commonly used. Measuring the solids
concentration of RAS allows the return volume to be
regulated to keep the solids level in the aeration basin within
the control parameters. Excess sludge which eventually
cumulates beyond that rebounde d is specified as Surplus or
Waste Activated Sludge (SAS/WAS). This is removed from the treatment process to keep the ratio of biomass to food
conveyed (sewage or wastew ater) in balance [8]-[10].
E. Maintaining the Proper Mi xed Liquor Concentration
The activated sludge process is a physical/ biological
wastewater treatment process th at uses microorganisms to
segregate wastes from water and to facilitate their
decomposition. When the microorganisms in activated
sludge come into contact with wastewater, they feed and
grow on the waste solids in th e wastewater. This mixture of
wastewater and microorganisms is known as mixedliquor.
As the mixed liquor flows into a secondary clarifier, the
organism’s activity slows and they begin to aggregation
together in a process known as bio-flocculation i.e. the
ability of one flock particle to stick to another. Because the
velocity of the water in the s econdary clarifier is very low,
the flocculated clumps of organism settle to the bottom of
the clarifier (as sludge), while the clarified water currents
over a weir. The settled organisms are constantly pumped
back to the front of the aeration laver to treat more waste
[8]-[10].
This is called reflux activated sludge, or RAS, pumping.
The clarified effluent is typically deodorized and then
discharged from the facility. As the organisms in the
aeration basin capture and tr eat wastes they grow and
rehabilitate and more and more organisms are created. To
function efficiently, the mass of organisms (solids
concentration) needs a steady balance of food (wastewater
solids). If too many organisms are allowed to grow in the
aeration basin, there will not be sufficient food for all of
them. If not adequate organisms are present in the basin,
they will not be able to consume the available food and too
much will be lost to the effluent in the form of BOD and
TSS. This balance between th e available food (F) and the
mass (M) of microorganisms is explained as the F:M ratio
of the system. The job of an activated sludge wastewater
treatment plant operator is to hold the correct mass of
microorganisms for th e given food supply . Because the food
supply does not typically change very much (that is, the
amount of wastewater solids usually stays the same from
day to day), operators must regulate the mass of organisms
that are allowed to agglomerat e in the aeration basin. This
adjustment is constructed by removing or wasting
organisms out of the system. Sludge that is intentionally
eliminated from the activated sl udge process is referred to
as waste activated sludge, or simply as WAS. Activated
sludge provides treatment through the oxidation and
dissociation of soluble organics and finely divided
suspended materials that were not removed by previous
treatment. Aerobic organisms carry out the process in a
matter of hours as wastewater flows through the aeration
tank and secondary clarifier [6], [10]. The organisms
stabilize soluble organic material through partial oxidation
resulting in energy for the organisms and by-products, such
as carbon dioxide, water, su lfate and nitrate compounds.
Finely divided suspended solids such as colloids are snared
during bio-flocculation and thus removed during
clarification. Conversions of dissolved and suspended
material into settle able solids as well as oxidation of
organic substances (digestion) are the major objectives of
the activated sludge process. High rate activated sludge International Journal of Environmental Science and Development, Vol. 5, No. 1, February 2014
84
systems tend to treat waste through transformation of the
dissolved and settle able solids while low-rate processes rely
more upon oxidation of these solids into gasses and other
compounds. Oxidation is carried out by chemical processes,
such as direct oxidation from the soluble oxygen in the
aeration laver, as well as through biological processes [7]-
[10].
Microorganism capture much of the dissolved organic
solids in the blended liquor rapidly (minutes), however,
most organisms will need a long time to metabolize the food
(hours). The concentration of organisms increase with the
waste load and the time spent augmentations in the aeration
tank. To sustain favorable conditions, the operator will
remove the excess organisms (waste sludge) to sustain the
required number of workers fo r effective treatment of the
waste [9], [10]. The mass of organisms that the operator
maintains is a function of the mixed liquor suspended solids
(MLSS) concentration in the aeration basin. By lowering the
MLSS concentration (increased wasting), the operator can
decrease the mass of organisms in the system. This
effectively elevatio ns the F:M ratio of the system. By
enhance the MLSS concentration (reduced wasting), the
operator can augmentation the number of organisms in the
system available to provide treatment. This has the effect of
lowering the F:M ratio. Again, controlling the rate of sludge
wasting from the treatment pro cess is one of the significant
control factors in the activat ed sludge system [7]-[10].
F. Measure importance of MLLS
If MLSS content is too high
–The process is prone to bulking and the treatment
system becomes overloaded
–This can cause the dissolved oxygen content to drop
with the effect that organic matters are not fully degraded
and biological 'die off‘
–Excessive aeration required which wastes electricity
If MLSS content is too low
–The process is not operating efficiently and is wasting
energy.
V. C
ONCLUSION
In the last years, new treatment methods such as chemical
oxidation, granular activated carbon adsorption, powdered
activated carbon treatment, wet- air oxidation, and anaerobic
treatment methods have been improved for the treatment of
wastewaters containing refractory compounds.
Based on the knowledge about the effectiveness of
microorganisms in the bioremediation of persistent
organics-contaminated soil and especially groundwater
environments by increasing the bioavailability of these
contaminants to microorganisms, it was considered that
bacteria might have an enhancement effect on the
biodegradation of persistent organic contaminants in
industrial wastewaters.
Biological treatment methods have been often considered
as the most complete, environmentally acceptable and cost-
effective treatment options. Th e presence of refractory or
toxic pollutants in the wastew aters often hinders treatment of these wastewaters through the biological processes.
R
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[7] E. Smulders, W. von Rybinski, E. Sung, W. Rähse, J. Steber, F.
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[9] A. Sacramento, “Operation of wastewater treatment plants,” Office of
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Batool Ahansazan was born on Aug 10, 1983,
Tehran, Iran. She was graduated of Sciences at
Valiasr Technical College of Tehran. Chemical
Industries Tehran, Iran, 1998-2003. She is student in
the master's degree in Energy Engineering at Science
and Research Branch, Isla mic Azad University(IAU)
She is a LECTUERER in Valiasr Technical College
of Tehran, Iran. Her research in the field of
nanotechnology and water treatment systems.
Ms. Batool Ahansazan is a member of Iranian Association of Chemical
Engineers and Iranian Society of Mechanical Engineers.
Narges Ahansazan was born on March 21, 1987,
Tehran, She was graduated of Sciences at Islamic
Azad University (IAU), Industrial engineering,
Tehran, Iran, 2005-2010.
She is an Industrial engineer in Arian Distribution
Company as a Supply Chain Manager. She is a
member of Iranian Association of Industrial
Engineers.
Zahra Ahansazan was born on June 26, 1989,
Tehran, Iran. She is student Electrical engineering,
electronicstrends at Islamic Azad University (IAU).
She is Electronics Engineer in Limenics Company.
She is a member of Iranian Association of electronics
Engineers.
Hossein Afrashteh was born on Aug 19, 1986,
Tehran, Iran. He was graduated of Sciences at Islamic
Azad University (IAU), Industrial engineering,
Tehran, Iran, 2005-2011.
He is an Industrial engineer in Afra Sanitary
Faucets. He is a member of Iranian Association of
Industrial Engineers.
International Journal of Environmental Science and Development, Vol. 5, No. 1, February 2014
85
, vol. 90, pp 1261-1268, 2009.
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